1. Field of the Invention:
The present invention relates to a false color preventing circuit for preventing false color from occurring in a color picture signal which is obtained through a color separating filter due to the high intensity of incident light.
2. Description of the Related Art:
Single-tube or single-plate type color imaging devices generally produce R, G, and B video signals or Y, R, and B video signals. However, a false color signal is generated when the incident light has a high intensity, because the intensities of incident light at which light sensing elements are saturated differ due to the difference in the transmittance of the color filters provided in the imaging device.
It has therefore been proposed to cancel the false color signal generated in the high-intensity component using a luminance signal which is in phase with the false color signal. This method, however, suffers from a problem in that the false color is left uncancelled on the periphery of the cancelled portion due to (a) difference in the frequency characteristics between the luminance signal and the color signal, (b) slight phase difference between these signals or (c) flare. Accordingly, Japanese Utility Model Publication No. 61-34793 discloses a method of cancelling a false color signal using a cancelling signal having a larger width than that of a luminance signal which is in phase with the false color signal.
FIG. 1 shows a conceptual structure of such a conventional cancelling technique, and FIG. 2 is a timing chart of the operation of the conventional cancelling device.
In FIG. 1, the output signal of an imaging device 1 is amplified by an amplifier 2, and the output of the amplifier 2 is delivered to both of a low-pass filter 3 and a color separation circuit 4 which respectively produce a luminance signal and color-difference signals. The resultant luminance signal and color-difference signals are supplied to a modulator 5 to produce a chrominance signal. Further, the output signal of the amplifier 2 is input to a comparison circuit 6 and a delay circuit 7. The comparison circuit 6 compares the two signals and outputs a signal having a higher level. In other words, the comparison circuit 6 outputs an inclusive or signal of the two signals input.
The output of the comparison circuit 6 is used as a cancelling (suppressing) signal of a cancelling circuit 8 to which the chroma signal is input. The chroma signal which has passed through the cancelling circuit 8 is input to an adder 9 where it is added to the output signal of a low-pass filter 10, and the resultant signal is output through a buffer 11 as a color video signal.
Assuming that an original signal a which is output from the amplifier 2 is the one shown in FIG. 2a, the comparison circuit 6 produces a signal c shown in FIG. 2c from the signal a and a delayed signal b which has passed through the delay circuit 7.
The output signal of the amplifier 2 passes through the low-pass filter 3 and the color separation circuit 4, and is input to the modulator 5 which produces a chroma signal. FIG. 2d shows a portion d of the chroma signal in which a false color is generated. Assuming that a signal which is in phase with the chroma signal is represented by a signal e, the signal e is delayed from the original signal a by a time t1 because of the color processing including color separation and modulation.
The comparison circuit 6 produces a cancelling signal c having a width larger than that of the luminance signal e by a margin of t2 in the positive direction of the time base and by a margin of t1 in the negative direction. The saturation component of the false color portion in the chroma signal is cancelled using this cancelling signal in the cancelling circuit 8. The resultant signal is added to the luminance signal which has passed through the low-pass filter 10 in the adder 9, and the added signal is output through the buffer 11.
Although the delay time t1 required for a color signal to be processed differs depending on the circuit structure or the method employed, it is mainly caused by a band-pass filter. About 1 .mu.s of delay time may occur at a maximum. This often makes the margin t1 of the cancelling signal produced in the conventional false color cancelling technique too wide relative to the false color signal, causing a normal color signal portion located at one side of the chroma signal containing false signal to be cancelled, as shown in FIG. 3.
In the case where the time duration of a high-intensity signal is very short, the ratio of the width of a false color signal to that of the high-intensity signal becomes very large, because, if an image sensor with a mosaic color filter, for example, is employed, a false color occurs at the front and rear edges of the luminance signal due to Moire caused by the color frequency of the filters.
It has therefore been considered to synchronize a cancelling signal having a large width from the original luminance signal such that it has minimum margins t2 and t2' at the two sides of the false signal. In FIG. 17, a delay 7' has a longer delay than delay 7. This technique is illustrated in FIG. 4 where output b of delay 7' corresponds to FIG. 4(c) and output c of delay 7 corresponds to FIG. 4(d). Reference symbols a and b in FIG. 4 respectively denote an original luminance signal and a chroma signal containing a false color. Signals c and d shown in FIGS. 4c and 4d are combined with each other to produce a cancelling signal shown in FIG. 4e. However, the resultant cancelling signal has no central portion, and is therefore insufficient to be used to cancel the false color. This failure caused by the cancelling signal is illustrated in FIG. 5. As is seen in FIG. 5, the closer to the apex of the triangle, the more the corresponding time length of the horizontal direction becomes shorted and the corresponding time length to draw line b is shorter than the time to draw line a. In this section x.sub.2, the corresponding chroma signal period is shorter than the lag time between FIG 4(c) and FIG. 4(d). The waveform mixed in comparison circuit 6 of FIG. 17 is, as shown in FIG. 4(e), a pulse waveform in which no signals are generated in period t.sub.x. This portion is the portion in which a corresponding cancelling pulse cannot be generated although a false color is generated.